The present invention relates to a magnetic disc device in a data processing device, and in particular to a magnetic disc device which incorporates a thin spindle motor having an enhanced shock resistance.
Magnetic disc devices for data processing systems have capacities which have recently become larger and lager, and recording densities which have become higher and higher. In particular, as to a magnetic disc device used in a personal computer, an electronic camera or other portable electronic devices, its portability is important, and further, a thin thickness, a shock resistance, low noise and low power consumption are required therefore.
By the way, a conventional spindle motor for driving a disc has used therein ball bearings so as to maintain precise rotation in order to aim at increasing the capacity of a magnetic disc device and at heightening the data recording density. However, the spindle motor using ball bearings, for driving a magnetic disc has a limitation on enhancing the rotational precision. Accordingly, there has been proposed a spindle motor using hydrodynamic fluid bearings, for driving a magnetic disc, as disclosed in Japanese Laid-Open Patent No. H6-223494, Japanese Laid-Open Patent No. H10-267036 and Japanese Laid-Open Patent No. H10-146014. That is, by using the hydrodynamic fluid bearings, it is possible to aim at enhancing the rotational precision and carry out axial positioning with the use of a magnetic attraction of the motor in view of requirement for the attaching posture and the portability of the motor.
Further, a magnetic disc device incorporated in a personal computer or an electronic camera, would be accidentally exerted thereto with an impact force upon dropping during attachment or portage, and accordingly, it is required for the magnetic disc to maintain the ability of recording or reproducing of data even though an impact is exerted thereto. The shock resistance for ball bearings is about 500 G. Note type personal computers in these days cannot cope with a shock resistance up to 1,000 G (upon dropping) if bearings other than ball bearings are used therein.
In the hydrodynamic fluid bearings, a herringbone groove for inducing hydrodynamic pressure is typically formed in a rotary shaft or a baring surface, and accordingly, the rotary shaft is supported by a fluid pressure which is produced on a bearing surface by rotation of the shaft. Further, the Japanese Laid-Open Patent No. H6-223494 or the Japanese Laid-Open Patent No. H10-267036 discloses such an arrangement that two bearings are normally used so as to precisely rotate a rotary body. Further, the Japanese Laid-Open Patent No. H10-146014 discloses a relationship between the inner diameter D and the length L of a bearing is set to L/D greater than 1 in order to precisely support a rotary body with the use of a single bearing, so as to have a required bearing stiffness. If the above-mentioned bearing device is used for a spindle motor for driving a magnetic disc, it is possible to obtain rotation with higher precision, and to aim at enhancing noise reduction and shock resistance in comparison with a motor using ball bearings.
By the way, as to a magnetic disc device mounted in an electronic device, the posture and the handling of the device cannot be specified, and accordingly, these facts should be taken into consideration for this magnetic disc device. Further, note type personal computers have recently been thinner and thinner, and accordingly, the housing of the device is required to have a height of 12.5 to 9.5 mm or 6.5 mm. Accordingly, if conventional dynamic pressure bearings are used, a required bearing stiffness cannot be obtained for these devices due to restriction on axial dimensions.
That is, it is required for a spindle motor for driving a magnetic disc, that the deflection of the rotary shaft of a bearing is restrained to be less than 0.5 xcexcm so as to precisely support the magnetic disc. By the way, since the height of the housing of the device is low, that is, 12.5 to 9.5 mm or 6.5 mm, the axial height of a bearing part has to be a value in a range of 3 to 5 mm or 3 to 6 mm. With such dimensions of the bearing part of the bearing device, the length of the bearing is small so that a required bearing rigidity cannot be obtained, and accordingly, two hydrodynamic fluid bearings can hardly be arranged in the housing. In particular, if the height of the housing is 6.5 mm, no thin bearings which can cope with such a height, are commercially available even though ball bearings are to be used. Thus, bearings having special specifications have to be used, and as a result, there would be caused such disadvantage that not only the cost would be increased but also the shock resistance would be lowered.
Further, in a magnetic disc device attached to a personal computer, a magnetic disc is fixed in a center clamp system by means of a rotary shaft as will be detailed hereinbelow. Accordingly, the minimum diameter of the rotary shaft in these magnetic disc devices becomes larger than 3 mm in view of the relationship between a bearing load exerted to the magnetic disc, and dimensions of clamp screws. Further, it is required to restrain the radial gap of the bearing from exceeding 2 xcexcm so as to decrease the static inclined angle of the rotary shaft in view of restriction to the dimensions of spacing between a magnetic disc and a head for reading and writing date. Meanwhile, in a conventional shaft support mechanism in which vibration is restrained by means of two radial bearings, a rotary shaft has to have a diameter larger than at least 4 mm in order to obtain a required the stiffness of shaft supporting if the height of the housing is low, that is, 12.5 to 9.5 mm or 6.5 mm. However, in such a conventional shaft support mechanism, if the minimum diameter of the rotary shaft is set to a value in a range from 3 mm to 4 mm, the bearing loss is increased, and accordingly, this mechanism can hardly be used for a personal computer or other electronic equipment.
Further, if the height of the housing of the bearing device becomes 5.9 mm or 5 mm, the sealing of the bearing device is difficult with the use of a single seal part due to the dimensional restriction, and accordingly occurrence of leakage of lubrication oil filled in a spindle motor cannot be prevented. In particular, since no contamination is allowed for a magnetic disc in a magnetic disc device, it is necessary for a spindle motor using hydrodynamic fluid bearings, for driving a magnetic disc, to take care of oil leakage, and accordingly, a highly reliable seal is required.
Meanwhile, if hydrodynamic fluid bearings are used, no noise occurs from the bearings. However, in a motor disclosed in the Japanese Laid-open Patent No. H10-267036, the magnetic center of a motor stator is shifted from that of a magnet for driving the motor so as to effect axial pressurization, and accordingly, there is a possibility of occurrence of electromagnetic sound. Further, in a motor disclosed in the Japanese Laid-Open Patent No. H6-223494, the axial positioning is carried out at a thrust bearing surface so as to align the magnetic center of the motor, and a ring-like magnetic plate is arranged at a position opposing an end face of a motor driving magnet so that axial pressurization is effected by thus obtained magnetic attraction force, thereby sustantialy no magnetic sound is produced. With this arrangement, if a thin motor is used, a gap between the end face of the motor driving magnet and the magnetic plate would be 0.1 mm or more or less, and accordingly the control of the gap is difficult. As a result, no constant magnetic attraction force is possibly obtained. Thus, even the hydrodynamic fluid bearing incurs the above-mentioned problems when it is used in a thin magnetic disc device.
The present invention is devised in view of the above-mentioned problems inherent to the conventional technology, and an object of the present invention is to propose a spindle motor which can effectively utilize the features of hydrodynamic fluid bearings, and which is thin and shock-resistant, and which exhibits a low noise and low power consumption, and a magnetic disc device utilizing the spindle motor, so as to provide a personal computer and other electronic devices which can use hydrodynamic fluid bearings and which excellent in portability.
According to a first aspect of the present invention, there is provided such an arrangement that a rotary shaft is rotatably supported by a hydrodynamic fluid radial bearing and a hydrodynamic fluid thrust bearing as first vibration restraint means, the radial bearing supporting a shaft translation mode of vibration components due to unbalance forces exerted to the rotary shaft, and the thrust bearing supporting a conical mode, thereof in order to maintain rotation with a high degree of precision. That is, a moment load caused by an unbalance force of a rotary body is balanced with a moment rigidity caused by hydrodynamic pressure induced by the thrust bearing so as to restrain vibration of a conical mode, and vibration in a translation mode is supported by the radial bearing. Thus, the relationship between the length L and the inner diameter D of the bearing is set to L/D less than 1 in order to precisely support a magnetic disc.
It is noted here that the bearing width L is an effective bearing width measured in the axial direction of the rotary axis at a bearing surface which carries out bearing action, and excluding the dimensions of chamfered parts in the end parts thereof. Further, the bearing inner diameter D is a half of the bearing surface which carries out bearing action and which is measured in the radial direction.
According to the present invention, loads can be received by the above-mentioned radial bearing and the thrust bearing, and the inner diameter of the hydrodynamic fluid radial bearing is set to a value in a range from about 4 to 5 mm in view of the bearing stiffness of the thrust bearing and a loss in the radial bearing while the relationship between the inner diameter D of the bearings and the length L is optimized to L/D=2.0 to 0.5 in order to precisely support the magnetic disc while reducing the bearing loss.
That is, in a conventional magnetic disc device, the diameter of the rotary shaft has been determined in view of a bending stiffness of a shaft and a loss of a radial bearing. In the motor according to the present invention, the diameter of the rotary axis is determined in view of a moment load caused by an unbalance of the rotor and a moment stiffness of the thrust bearing. Accordingly, in the magnetic disc device, according to the present invention, the relationship between the inner diameter D and the length L of the radial bearing is set to L/D less than 1 in order to maintain precise rotation in combination of the thrust bearing.
In addition to these matters, the moment stiffness of a thrust bearing is used, and accordingly, the length L of the radial bearing is reduced to a value in a range of 1 to 2 mm in view of a shock load and a bearing loss so as to reduce the bearing loss without hindering a bearing function, and further, the inner diameter D of the radial bearing is increased to a value in a range from 5 to 4 mm so as to increase the moment stiffness of the thrust bearing in order to cope with lowering of the rigidity of the radial bearing caused by a decrease in the length of the bearing, thereby it is possible to maintain precise rotation and to reduce the bearing loss.
Further, in addition to the first means, magnetic fluid is used as lubrication oil for the bearings while a rotary shaft which is magnetically permeable (made of a material having a high magnetic permeability or a magnetic material), and radial and thrust bearings which are magnetically permeable or which are made of a magnetic material are used while a permanent magnet is located between the radial bearing and the thrust bearing. Accordingly, the bearing and the magnetic fluid are magnetized so as to hold the magnetic fluid at the slide surfaces of the bearings. Thereby it is possible to ensure lubrication and to effect a magnetic attraction force which can prevent the magnetic fluid filled in the bearing device from leaking from the bearing device.